This paper describes the performance of the optical image stabilizer system developed
for the franco-Italian T.H.E.M.I.S. telescope, which is currently being built
on the Canary Island Tenerife. T.H.E.M.I.S will be one of the largest solar telescopes,
and will be the first to be manufactured with a real time tip-tilt correction
system integrated into its optical train. A new method is described here for
measuring image motions. This method is called granulation tracking. It is based on Fourier
transform analysis. A servo-loop model is also proposed, based on experimental
measurements and the known time properties of atmospheric turbulence, to deduce the image
quality obtained after correction. Method sensitivity and accuracy under various terrestrial
turbulence conditions are discussed.